1. Field of the Invention
The present invention relates to methods of processing seismic data. In another aspect, the present invention relates to a method for datuming seismic data. In still another aspect, the present invention relates to a method for processing seismic data by datuming the seismic data and then applying surface consistent corrections.
2. Description of the Related Art
Seismic exploration generally involves generating seismic pulses at the surface of the earth by means of one or more seismic sources. The seismic pulses travel downwardly into the earth with a fractional amount being reflected and/or refracted due to differences in elastic properties at the interface of various subterranean formations.
Detectors, such as seismometers, geophones or hydrophones, produce analog electrical seismic signals or seismic trace signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals or seismic trace signals from the detectors can then be recorded. Alternatively, the analog seismic signals or seismic trace signals from the detectors can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
From the recorded data, a seismic section is generated. A seismic section is a seismic image depicting the subsurface layering of a section of earth along a seismic line of profile. The seismic section is an important tool which the geologist studies to determine the nature of the earth's subsurface formations. However, before an array of seismic samples can be converted into a seismic section which can be interpreted by the geologist, the seismograms must be processed to reduce the degradation due to noise.
Seismic interpretation generally involves the study of the behavior of arrival times, amplitudes, velocities, frequencies, and character of the reflections from target horizons. Any changing or anomalous behavior is of particular interest. Consequently, we require the zones of interest to be free from disturbances generated elsewhere, such as those caused by the surface and near-surface layers, subsurface horizons, subsurface anomalies, and by the energy source and field recording system.
It is known that near-surface features create not only a time shift or amplitude decay, but also a more complicated frequency dependent, time-varying filtering effect. This unwanted effect is not confined to the near-surface part of the section, but instead is present throughout the data, as energy reflected from deeper layers passes through the near-surface twice while traveling from source to reflector to receiver.
This unwanted effect can distort or even ruin the seismic data by obscuring or masking seismic events related to the reflections and/or refractions from the subterranean formations.
Prior art methods for correcting seismic data for such near-surface effects exist. For example, see Taner et al., "Surface Consistent Corrections", Geophysics, v. 46, pp. 17-22, and Taner et al., "Estimation And Correction Of Near-Surface Time Anomalies", Geophysics, v. 39, pp. 441-463. Such methods assume that the geologic conditions which effect the data are at the surface.
Unfortunately, geologic conditions other than those at the surface can cause the distortions discussed above such as time shift or amplitude decay, and the more complicated frequency dependent, time-varying filtering effect. As with near surface anomalies, the effect is not confined at or near the particular geologic condition, but rather is present throughout the data. Surface consistent corrections, which are designed to correct for geologic conditions at or near the surface, fail to correct for such subsurface anomalies.
U.S. Pat. No. 4,577,297, issued Mar. 18, 1986 to Kalkomey et al., discloses a method for enhancing recorded seismic reflection signals having undulating water bottom distortions. In this method, simulated seismic reflection signals are utilized to reposition the shots and receivers to a fictitious plane located below the water bottom.
U.S. Pat. No. 4,887,244, issued Dec. 12, 1989 to Willis et al., addresses the problem of data insufficiency in seismic traces, most commonly caused by lack of recorded data or noise. The insufficient data is interpolated using a forward and backward application of the wave equation to create a fictitious datum plane.
As discussed, both U.S. Pat. Nos. 4,577,297 and 4,887,244 rely upon the creation of a fictitious seismic plane which may not correspond to an actual physical subsurface event.
Therefore, there exists a need for a technique for eliminating effects of subsurface anomalies without the limitations of the prior art.